There are three objectives. The first objective is to determine if the Gouy-Chapman-Stern theory of theaqueous diffuse double layer describes adequately the interaction of tetravalent cations, such as gentamicin, with phospholipid bilayer membranes. The results will provide a new test of a fundamental physical theory and will be useful to clinicians and pharmacologists studying the interaction of aminoglycoside antibiotics with phospholipids. Recent work suggests that phospholipids are the "receptor sites" responsible for the nephrotoxic effects of gentamicin. The second objective is to determine the electrostatic potential adjacent to membranes when the fixed charges are located a Debye lenght or more from the interface. Membranes will be formed from mixtures of the zwitterionic lipid phosphatidylcholine, PC, and the glycolipid GM1 to mimic the electrostatic properties of biological membranes. GM1 has one negative charge, which is located about 1 nm from the surface of the membrane. The zeta potential of both an erythrocyte and a PC:GM1 vesicle are about 1/3 the value predicted from the Gouy-Chapman-Stern theory. These measurements will provide the first critical test of three new theories that puport to describe the electrostatic potential adjacent to biological membranes. For these two projects zeta potentials will be calculated from microelectrophoresis measurements made on multilamellar vesicles, erythrocytes and VSV virions, the change in the potential within a membrane will be determined from kinetic conductance measurements made on planar phospholipid bilayers in the presence of lipid soluble ions and carriers, and the change in the potential above a monolayer will be measured directly with an ionizing electrode. The third objective is to test a new hypothesis about the reabsorption of fluid from the renal proximal tubule. The theory postulates that Helmholtz-type electro-osmosis moves fluid through the lateral intercellular spaces of this epithelium. The theoretical aspects of this project will be developed in collaboration with Dr.R. Mathias (Rush). The hypothesis will be tested experimentally by measuring, under various experimental conditions, the zeta potentials of vesicles formed from basolateral membranes isolated from a variety of epithelia.